Advances in our understanding of the structure,
composition, dynamics and evolution of the Earth's atmosphere
have come about because of studies conducted by scientists
and engineers from a broad range of disciplines, including,
among others, meteorologists, spectroscopists, and
physicists. Principles from physics, chemistry and
mathematics are used in all aspects of atmospheric
research. Since these are subjects that are also fundamental
to the education of chemical engineers, many chemical
engineers have chosen careers aimed at solving atmospheric
problems. One focus has been on understanding of climate
change, in part because of the strong link between climate
change and the Earth's chemical composition.

In this section we briefly describe the structure of the
Earth's atmosphere, how some simple chemical engineering
principles are used to detect and describe changes in the
atmospheric chemical composition, and how pollutants are
believed to affect the Earth's climate.

Structure of the atmosphere

The figure below shows a schematic of the Earth's
atmosphere that includes a sketch of the variation in air
temperature with height. The lowest layer of the atmosphere,
in which we live, is called the troposphere. Temperature
decreases with height in the troposphere, as anyone who has
flown on an airplane will have noted. The troposphere is also
the layer in which "weather" is experienced: clouds, rain and
snow precipitation, hurricanes and tornadoes, and
thunderheads and lightning are all observed in the
troposphere. You may have observed the formation of a
thundercloud and noted that the very high top of the cloud
"flattened out", or appeared to "bubble over". This happens
when the cloud motions are so vigorous that the cloud grows
through the depth of the troposphere and hits the barrier
known as the tropopause. At the tropopause, the dependence of
air temperature with height is reversed: temperature begins
to increase with altitude. This type of temperature profile
is called a stable profile, because rising air parcels lose
bouyancy - the air surrounding them is warmer than they are,
and they tend to sink instead of rise. (Think about what
happens if you add cold cream to a cup of hot coffee.) The
tropopause thus acts like a "lid" for the troposphere and
prevents mixing of air across it into the next layer, the
stratosphere.

The layers in the atmosphere can be identified by the strong variations in the temperature profile.

The stratosphere is characterized by an increasing
temperature profile with height, called an "inversion", and
very stable, turbulence-free air. The stratosphere is also
very dry because moisture that is evaporated from the Earth's
surface from oceans and land cannot rise past the lid
presented by the tropopause. As shown in the figure,
commercial and supersonic aircraft fly in the tropopause /
lower stratosphere regionsabove the "weather" because of
the reduced drag from the thin air and the low
turbulence. The cause of the stratospheric temperature
inversion is the stratospheric ozone layer, concentrated near
25 km altitude. Ozone (O3) absorbs ultraviolet
solar radiation, in turn raising the temperature. Although
ozone that forms near the surface of the Earth in polluted
regions is considered harmful to human health, the
stratospheric ozone layer is extremely beneficial: it helps
maintain the troposphere / stratosphere temperature
inversion, and it absorbs solar ultraviolet radiation before
it can reach the surface and cause damage to biological
organisms, including skin cancer in humans.

Global mass balances

Two of the simplest, yet most useful, engineering
principles that are applied to the atmosphere are the
concepts of mass and energy balances. To illustrate the first
type of balance, imagine a leaky bucketone with a hole
near its bottom. If the bucket is initially filled with
water, the water level drops as water leaks out of the
bucket. Water could be supplied to the bucket by placing it
under a tap and turning on the faucet. If the flow rate from
the faucet is adjusted to exactly match the leak rate, the
water level remains constant. If the faucet supplies water at
a slower or faster rate, then the water level continuously
drops or rises, and the rate of change of the water level
depends upon how different the supply and loss rates are.

Although this example is extremely simple, it illustrates
the basic idea behind constructing budgets for different
chemical components in the atmosphere. We need to measure the
rate at which the component enters the atmosphere (emission
rate), the rate at which it leaves the atmosphere (sink
rate), and its current level (atmospheric concentration)
along with the rate of change of that level, which can be
zero if the budget is "balanced"  i.e., if the emission
rate equals the sink rate. Let's examine the global budget of
carbon dioxide, CO2, as a relevant example. Carbon
dioxide is taken up from the atmosphere by plants during
their growth cycle, when they convert CO2 and
water into oxygen and biomass. Some carbon dioxide can also
be dissolved into the world's oceans, although this is a
rather slow process. These two processes are the major sinks
for atmospheric CO2. The sources of CO2
include the decay of dead plants, natural combustion
processes (e.g., burning of forests by wildfires), and
combustion processes due to human activity, which include
gasoline burning, coal burning to produce power, and burning
to clear land for agricultural activities. The natural
emissions and sinks are ten times larger than the source due
to human activity, and are approximately in balance.

The figure below shows the atmospheric concentration of
CO2 from about the year 1000 A.D. to the
present. (The early data are obtained from ice cores that are
drilled from polar regions. Bubbles of air trapped in the ice
at various depths are then analyzed for atmospheric chemical
composition during the era represented by that ice section.)
The notable feature in the timeline is the sharp increase in
atmospheric CO2 concentrations after about the
mid-1800's. Prior to this time, the global cycle must have
been approximately in balance, since atmospheric
concentrations did not change significantly. The timing of
the sudden increase coincides with the start of the
Industrial Revolution, when for the first time in history
large amounts of fossil fuels (coal, oil and natural gas)
were burned for powering engines and producing
electricity. The result was that the number of sources of
CO2 to the atmosphere was abruptly increased,
throwing the budget out of balance and, over time, causing a
dramatic increase in atmospheric concentrations of
CO2; the sources exceeded the sinks.

Atmospheric CO2 concentrations over the past 1000 years from the recent ice core record and (since 1958) from a measurement site on the Mauna Loa volcano. The solid curve is based on a 100 running average.

A more detailed look at changes in CO2 is given
in the following figure, which shows data from Mauna Loa in
Hawaii from the late 1950's to the present. There is a
pronounced annual cycle in CO2 concentrations when
viewed at a single site, rather than as a global average. At
Mauna Loa, this is due to the annual cycle of growth and
decay of plant life in land masses north of the equator; the
air affected by these sources and sinks is sampled at the
Mauna Loa site. If only those natural sources and sinks were
present, the annual cycle would remain in the data, but the
overall trend of the data would be flat  that is, the
average would not increase over the decade from 1960 to the
present. Instead, the small (~10%) annual perturbation in the
budget that is due to fossil fuel burning in the Northern
Hemisphere constitutes a net, unbalanced source that produces
the observed upward trend with time.

The CO2 concentration in the atmosphere measured at Mauna Loa, Hawaii, since 1958, showing trends and seasonal cycles.

The global energy balance

The figure below shows an accounting of what happens to the
energy from the Sun that strikes the Earth. Because the
Earth's overall energy budget is in balance, the energy
leaving must equal the energy coming in (about 343 watts per
square meter, or W m2). However, the form of
the energy does not necessarily have to be the same entering
or leaving, and indeed it is not the same because of
interactions with the atmosphere. The energy from the Sun is
concentrated in the visible and ultraviolet part of the
energy spectrum. Some of this light energy (about 103 W
m2) bounces back to space when it hits the
atmosphere. The remainder hits the surface of the Earth and
is absorbed. It is re-emitted, but now as thermal energy
(longer-wavelength, or infrared, radiation) that eventually
leaves the atmosphere to complete the energy budget. Before
it leaves, however, it interacts with "greenhouse gases" in
the atmosphere that can absorb and re-emit energy with these
longer wavelengths (but that cannot interact with the
shorter-wavelength solar energy). The absorption and
re-emission have the effect of warming the surface and the
troposphere, creating the warmer temperatures that support
life as we know it.

A schematic of Earth's overall energy balance. The net input of solar radiation must be balanced by the net output of infrared radiation. About one-third of incoming solar radiation is reflected and the remainder is mostly absorbed by the surface.

What factors could change this energy balance? Consider
first changes in the amounts of greenhouse gasesa
subject that has been the focus of intense international
scientific and public interest. The most-discussed greenhouse
gas is carbon dioxide (CO2), which is a product of
any combustion process that burns carbon-containing
fuels. These include grasslands, wood, peat, coal, and
oil. Higher levels of CO2, created by increased
global industrialization, increase the absorption of infrared
radiation from the Earth's surface. However, the atmosphere
must still radiate 240 W m2 of thermal
energy to space. To fulfill this requirement, the temperature
rises, because emission of thermal energy increases with
temperature. This brings the energy budget back into balance,
but the net effect is that tropospheric temperatures are
increased when CO2 levels rise.

For a different scenario, consider next only the solar
radiation that is immediately reflected back to space. Clouds
and pollutants present in the atmosphere are responsible for
some of this reflection. If global cloudiness changes, or if
global pollution levels rise, then more than 103 W
m2 could be bounced back to space. Since the
total energy in must still equal the total energy out, that
leaves less solar radiation to penetrate to the surface, be
re-emitted, and interact with greenhouse gases. The overall
result would be a cooling of the surface and troposphere,
relative to today's temperatures.

Many scientists believe that some of this cooling has
taken place, and has counteracted global warming from
increases in greenhouse gases. The figure below, which shows
observations of global temperature compared with model
predictions, provides support for these ideas. In the top
panel, the observations since the late 1800's, shown by the
solid line, are compared against model predictions that
include only the effects of rising global greenhouse gas
concentrations. There is a range of model predictions because
there is still much uncertainty in how to correctly simulate
the many complex processes occurring in the
atmosphere. Nevertheless, most of the models predict more
warming than has been observed. In fact, such discrepancies
between observations and models has led to much skepticism of
their use in projecting climate change, and criticism of
basing international policy decisions related to greenhouse
gas emissions on such simulations.

A comparison of observed and modeled (predicted) global mean temperatures. The solid line in each plot represents the actual observed temperature, while the dashed lines indicate predicted values.

However, as can be seen from the middle and bottom panels
in the figure, the modeling approach itself is not completely
to blame: the model must be supplied with enough detail to
mimic all the processes that are important. A fuller
understanding of the various factors that can contribute to
global warming or cooling has helped improve the agreement
between observations and model predictions substantially. In
the middle panel, the cooling effects due to rising levels of
pollutants in the form of particles ("aerosols") has been
added to the greenhouse gas effects. This counteracting
cooling creates a modeled trend much closer to the global
temperature trend. In the bottom panel, both effects are
included along with simulated variations in the energy output
of the Sun, which has its own multiyear cycle. The solar
variations reproduce some of the fine structure in the
observational trend. The enhanced understanding that has made
such accurate simulations possible in recent years has come
about because of contributions from scientists and engineers
from many disciplines who have individually studied the
various components of the global energy budget and suggested
how these contribute to the overall "big picture".

Atmospheric particles and visibility

In the above discussion, pollution in the form of
atmospheric particles  also called "aerosols" 
was implicated in changes in the global energy budget because
of the particles' activity in reflecting incoming solar
radiation back to space. Particles in the atmosphere can
interact with light not only in this way, but also such that
they affect visibility. This is shown in the accompanying
figure. An observer perceives an object because light strikes
that object and reflects into the eye of the observer. The
reflected light carries information about the object,
including its color. Particles in the atmosphere interfere
with perception of the object in several ways. The light
reflected from the object can be scattered out of the
observer's line of sight, or absorbed before it reaches the
observer; the intensity of the perceived image is thereby
lessened. Sunlight can also be bounced into the observer's
line of sight from the atmosphere around the object; this
light carries no information about the object and thus just
serves to obscure the image (a similar effect as glare from
oncoming headlights that obscures vision of the road
ahead). Visibility is measured in terms of visual range, the
farthest distance that a dark object on the horizon can just
be perceived. The higher the concentration of particles in
the atmosphere, the shorter the visual range.

Contributions to atmospheric visibility include
light from a target (e.g., the mountains) reaching the observer, light
from the target scattered away from the observer's line of sight, and
sunlight scattering into the observer's line of sight.

What are the sources of atmospheric particles? There are
some very large, episodic sources that are natural in
origin. The first figure below shows a satellite image of a
dust storm in the Sahara that has lifted huge quantities of
soil particles into the mid-troposphere. Such dust storms
recur each year in Africa in the summertime, when
meteorological patterns create intense convective (lifting)
activity in the region and prevailing winds sweep the
aerosols across the Atlantic Ocean. Saharan dust is detected
annually in Bermuda, Barbados, and the southeastern U.S., and
has been shown to be responsible for episodes of poor
visibility in Florida. (Interestingly, the minerals in the
dust that falls into the Atlantic Ocean during transit are
important sources of nutrients to marine life.) There are
also large sources of dust in Asia during the springtime,
again when weather patterns favor lifting and transport
eastward, out across the Pacific. Asian dust is frequently
detected at Mauna Loa and occasionally along the West Coast
of the U.S. The second figure shows just how far the Saharan
dust can persist as it travels almost due west across the
Atlantic towards Central and North America.

This February 26, 2000 image shows a dust plume
obtained from TOMS (Total Ozone Mapping Spectrometer) data over the
Sahara Desert and extending over the Atlantic Ocean and Canary
Islands. The land sources of the dust plume are clearly visible, with
the main source coming from Western Sahara and Mauritania. The green
to red false colors in the dust plume image represent increasing
amounts of aerosol, with the densest portion over the ocean. Under the
densest portions of the dust plume (red) the amount of ultraviolet
sunlight is reduced to half its normal value, while over the land
(green) the UV sunlight is reduced by about 20%. The high dust amount
over the ocean was not present on previous days. Between February 27
to February 29 the ocean dust plume decreases while a massive dust
plume develops over the land that covers a region from the equator to
30o N latitude. Based on previous dust events observed by
TOMS, there should be another dense plume over the ocean during the
next few days.

A TOMS satellite image showing transport of
aerosols (i.e., dust) from their source in Saharan Africa, on July 10,
1999.

Another large, episodic, natural source of particles is
fires, usually initiated by lightning strikes during
thunderstorms. In some years, drought conditions encourage
frequent and fast-spreading fires that combust large numbers
of acres of forest or grassland. In some developing
countries, including in Central and South America and in
southern Africa, annual burning is used to clear cropland for
planting. The smoke particles released by fires create local
and sometimes long-range pollution problems. The figure below shows
a satellite image that has captured one such smoke event in
May 1998. Severe drought in Central America earlier that year
left the region susceptible to uncontrollable fires, which
began burning in early Spring and intensified in May. During
that time of the year, prevailing winds usually shift,
bringing air from Mexico into the southern U.S. This shift is
captured in the image: the fires are located in the Yucatan
and southern Mexico, and produced the heavy plume blowing
west into the Pacific. The shifting winds, however, rapidly
carried a secondary plume northward into the U.S., where the
smoke was detected as far north as Wisconsin. The May 1998
fires and accompanying smoke were so severe, and persisted
for so long, that Texas issued a public health advisory for
most of the month, warning people to limit the amount of time
spent out of doors.

A TOMS satellite image showing smoke transport across Central and North America, on May 18, 1998. Fires located in the Yucatan produced a plume which was carried as far north as Wisconsin.